Some questions/answers use imperial units instead of SI units. Here you are some examples:

Height between 62 and 75 inches

The skin is actually encountering 0 atmospheric pressure, and the suit is porous to allow sweat to escape, but maintains even mechanical counter-pressure to the 3 or so pounds per square inch needed to avoid vacuum injury

Launching out of Earth's gravity well is challenging. A 6.2 million pound Saturn 5 rocket could put ~260,000 lbs into low Earth orbit (4% of initial mass into orbit)

Should these values be rewritten to instead use SI units, since it's an international standard? If yes, is it possible to put them automatically in review queue, when some keywords are spotted?

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    $\begingroup$ In my humble opinion: yes. If user wants to keep imperial units, he/she should put it after SI value into parenthesis. $\endgroup$ – user55 Aug 18 '13 at 0:24
  • $\begingroup$ Are you reading my mind? Just wanted to ask the same ...... scary. $\endgroup$ – s-m-e Aug 18 '13 at 2:00
  • $\begingroup$ May I also suggest to expand your question in terms of quantities? We need to specify a decimal separator and stick with it ... $\endgroup$ – s-m-e Aug 18 '13 at 2:12
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    $\begingroup$ @ernestopheles perhaps we are the only two european guys here ;) $\endgroup$ – user55 Aug 19 '13 at 14:44
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    $\begingroup$ I'm definitely European, but as I'm British we use both metric and Imperial, being awkward is our thing. $\endgroup$ – Rory Alsop Aug 19 '13 at 20:59
  • $\begingroup$ @RoryAlsop Awkward indeed ;-) $\endgroup$ – s-m-e Aug 19 '13 at 23:15


In my humble continental European opinion, imperial units are painful. This equally applies to CGS (dear astrophysicists, just don't even bother to justify this system). Besides, imperial units even screwed up entire space missions. If this is not reason enough ...

It is my understanding that this is an international and professional forum. If we want to provide quality questions and answers, we do need to enforce one unique system. SI seems like the logic choice, from an academic point of view. On the contrary, we need to accept, that we grew up with different unit systems. So it is only fair to provide numbers in other unit systems, too, as an addition.

I do not like the idea of closing or down-voting questions or answers because of unit issues. Simply leave a comment or edit the original post by adding SI values.

There is a related question on meta.physics.SE.

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  • $\begingroup$ Was my question supposing a down vote? If so was not intended. $\endgroup$ – user55 Aug 18 '13 at 7:42
  • $\begingroup$ @trapo Ok, I understood your last comment. No, this was not your intention, but I just wanted to explicitly exclude this possibility in this discussion. $\endgroup$ – s-m-e Aug 18 '13 at 10:29
  • $\begingroup$ I thought it was mixed units that caused issues. If all lengths were measured in standard barley corns, all times in fortnights, all masses in standard grains, etc., then the specific problems would have been avoided. If programming systems managed unit information, errors could be even further reduced (and clarity of expression could be improved). (It is unfortunately that SI is base ten; if base 16 was used by humans, oddities like kibi, mebi, gibi, tebi, et al. could have been avoided. :-/) $\endgroup$ – Paul A. Clayton Aug 20 '13 at 22:10
  • $\begingroup$ @PaulA.Clayton Theoretically, mixed units are the primary issue, yes. However, SI units were 'designed' to be consistent - they work like a well behaving entity. To date, there is no other known system with a similar depth, completeness and behaviour. Every other existing system starts to suffer from strange conversions within at some point. If you dig for e.g. older American papers in electro-dynamics / magnetism, you will see a lot of strange inconsistent stuff. $\endgroup$ – s-m-e Aug 21 '13 at 9:02

Spaceflight legitimately uses a mixture of units, and thus, the units in question might vary significantly. As a result, I don't think it entirely makes sense to force a particular convention on how units are given. If NASA says an astronaut must have a "Height between 62 and 75 inches. ", then it doesn't make sense to force conversion of such units. Also, if they give the thrust of a rocket in pounds, the same goes.

The whole space community is fragmented about what units to use, and while there appears to be significant effort to use metric parts, it hasn't been fully adopted yet. As a result, forcing SI units might actually make it difficult to ask/answer certain questions.

However, if kind souls wish to edit answers to include SI units, that's great. Also, if the original poster wants to include them, it also make sense.

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  • $\begingroup$ If unit is taken from some other source, I think would be "wrong" to change to SI, but other in the other cases I would be glad to see SI or double notation. From my point of view, not having SI or double notation is something like unformatted code on SO: you can understand question but unformatted code slows you down. However, any decision this community takes, it would be great to have a FAQ entry like "What kind of units have I to use on this site?" $\endgroup$ – user55 Aug 19 '13 at 14:59
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    $\begingroup$ I would argue the 'legitimacy' of mixed units in professional environments. You can not compare values, and this is a massive problem. In the space community, the fragmentation is US vs non-US, as stupid as this may sound. From experience, there is no other country causing so much trouble in any cooperation when it comes to units and numbers, not even the UK. I regularly waste a lot of time on converting units and checking the conversions over and over again. It is a major source of error and I just hate it. $\endgroup$ – s-m-e Aug 19 '13 at 16:29
  • $\begingroup$ Ironically, your link on thrust / shuttle main engines provides all (except one) numbers in both, imperial and SI units. The first is honouring the traditions of how people grew up, which is ok, the second is for professional use. I would love to see the same here. (Weirdly, they give thrust in kg instead of Newton, but it is still an improvement over lbs.) $\endgroup$ – s-m-e Aug 19 '13 at 16:57
  • $\begingroup$ It's been my experience that the only times imperial units pop up is when (a) you're dealing with an old model/old code, or (b) you're speaking with an old dude. Every new project I've worked has dealt with SI numbers, and some of the more organized ones mandate them explicitly from the start. Just my two cents. $\endgroup$ – user29 Aug 20 '13 at 0:16
  • $\begingroup$ @Chris PIs, leading engineers ... a number of old dudes ... Yes, it is enforced, but there is always another 'internal study' or some report from an engineering company. There is stuff, which was designed with imperial units in mind although it is drawn in metric units - lots of weird decimal places ... SI is just not there yet. $\endgroup$ – s-m-e Aug 21 '13 at 9:15

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